Producing a clean synthetic liquid fuel from a gas including methane as a feedstock contributes to the effective utilization of untapped natural gas and other resources and to the supply of an environmentally-friendly clean energy source. Actually, however, technologies for producing synthesis liquid fuel have hardly been commercialized anywhere in the world at present, except for methanol that is used as the feedstock for the chemical industry. This is because the technology is not economically viable for commercial application based on the current level of technology and the prices of existing fuels, indicating that further improvement of the technology is indispensable for reducing the cost. This technology consists of two phases; the production of a synthesis gas from a gas that contains methane, and the production of a liquid fuel from the synthesis gas. Reduction of cost in the former phase that accounts for about 60% of the total equipment investment has a greater impact. With the background of concerns over shortage of natural resources and the global environment, there have been growing social demand for practical application of this technology, thus prompting the development of technologies that reduce the cost and improve the energy efficiency in the stage of producing the synthesis gas.
At present a steam reforming process is principally employed in the production of synthesis gas used as the feedstock for producing methanol or other purposes. In addition to the fact that steam reforming is an endothermic reaction that requires the supply of heat from the outside, an excessive quantity of steam is supplied in order to avoid a decrease in catalytic performance resulting from the precipitation of carbon as well as the carburizing metal dust in a waste heat recovery boiler installed downstream of the reactor, which result in a low energy efficiency and high investment cost. While a hydrogen to carbon monoxide ratio of 3 has been achieved for the synthesis gas produced by steam reforming, it would be necessary to add a large quantity of CO2 to the feedstock gas or partially separate hydrogen from the synthesis gas, in order to provide the synthesis gas that is suited for the production of liquid fuel, namely one that has the above-mentioned ratio of 2 or 1.
In order to overcome the drawbacks described above, especially the necessity to supply heat from the outside, there has been a progress in the development of internally heated reforming reactor technology that introduces pure oxygen directly into the gas flow which includes methane. According to this technology, the heat required for the reforming reaction is supplied from the combustion of the feedstock gas that takes place inside of the reactor, and is based on partial oxidation of methane that is represented by the following formula (standard enthalpy of formation is 298K).CH4+1/2O2→CO+2H2+35.5 kJ/molThis technology can be classified by the form of reaction into gas phase partial oxidation, fixed bed autothermal reforming, fluidized bed autothermal reforming and catalyst partial oxidation. Except for the catalyst partial oxidation that is said to have partial oxidation reaction proceed in a single stage, these processes include the methane combustion reaction and the reforming reaction of CO2, H2O that are generated by the combustion with methane. That is, the following exothermic reaction and endothermic reactions proceed in combination.CH4+2O2→CO2+2H2O+801.7 kJ/molCH4+H2O→CO+3H2−206 kJ/molCH4+CO2→2CO+2H2−247.5 kJ/molAmong these processes, the process that is being applied to commercial plants is the gas phase partial oxidation while there has not been much progress in the commercial application of the fixed bed autothermal reforming process. The fluidized bed autothermal reforming process remains at the stage of pilot testing, while the catalytic partial oxidation remains at the basic research stage. As these technologies utilize the thermal energy supplied from the inside, energy efficiency is improved over that of the steam reforming, but it is difficult to significantly reduce the cost because it requires the supply of expensive pure oxygen produced by the cryogenic distillation air separator.
Current technologies to produce oxygen, represented by the cryogenic distillation, are capital intensive and energy intensive, and have low prospect of significant cost reduction because they are intrinsically based on the difference in the boiling point of oxygen and nitrogen that have very low boiling points, in spite of some advancements that have been made so far including the improvement in the process configuration. With this background, a novel technology for producing oxygen that has been vigorously researched in Western countries in recent years is the high temperature (up to 850° C.) oxygen separation technology that uses a dense mixed conductivity ceramic membrane. This technology is regarded as very promising because it employs a compact facility and offers the possibility of reducing energy consumption. In a mixed conductivity ceramic membrane, oxygen molecules included in air are selectively ionized on the surface of the membrane and pass through the crystal lattice, being driven by the difference in partial pressure of oxygen on both sides of the membrane. At the same time, electrons move in a direction opposite to the flow of oxygen ions so as to maintain the electrical neutrality Partial pressure of oxygen on the methane side can be minimized by placing a catalyst on the side opposite to the air side so as to oxidize methane and other substances (consume oxygen), thereby maximizing the difference in partial pressure of oxygen, namely the driving force for the selective permeation of oxygen. As the ceramic material exhibits mixed conductivity at a high temperature from 800 to 900° C., partial oxidation reaction of methane and other substances can be carried out simultaneously with the separation of air, namely in a single unit, thus providing the possibility that the reactor could become more compact and low-cost. For these reasons, basic research on the use of the ceramic membrane has been conducted by many researchers, such as the methane oxidation coupling and production of synthesis gas from methane. Particularly intensive efforts have been poured, world-wide, into the production of synthesis gas from methane including a big research project with a fund granted by the U.S. Energy Department, since there are possibilities of drastic cost reduction and reducing energy consumption.
As the basic technologies upon which the synthesis gas producing technologies that employ a ceramic membrane reactor to produce synthesis gas from a methane-containing gas have been developed, materials for the mixed conductivity ceramic membrane, configuration of the membrane including thin membrane, methane reforming catalyst and the arrangement thereof, configuration of the reactor including the sealing method, reaction heat supplying method and the entire process including the membrane reactor have been disclosed.
For the properties of the mixed conductivity ceramic membrane material, material stability as well as high oxygen permeability, particularly stability of the crystal structure in the reducing atmosphere are of great importance. When a change in the crystal structure and/or the phase change occurs, the membrane often swells and this eventually results in the destruction of the ceramic membrane. Progress has already been made, world-wide, for the development of ceramic membrane materials that have high oxygen permeability and high material stability in the reducing atmosphere, such as Sr1.7La0.3Ga0.6Fe1.4O5.15 (WO99/21649) that has very high level of oxygen permeability and material stability in the reducing atmosphere.
As for the configuration of the membrane, it must be thin in order to increase the oxygen permeability against the bulk diffusion resistance of oxygen in the ceramic membrane, and such proposals have been made as a method for producing an inorganic thin film by means of organometallic chemical deposition on a porous supporting structure (Japanese Unexamined Patent Publication (Kokai) No. 6-135703). Also in case the surface exchange such as selective dissociation of oxygen reaches the rate-determining step of the oxygen permeation rate as the membrane is made thinner, it may be required to add a porous layer that increases the surface area or add an oxygen dissociating catalyst (WO98/41394, etc.).
As for the overall process that contains the membrane reactor for producing the synthesis gas from a gas including methane, proposals have been made on such aspects as the form of a process with various unit operations specified, and process running conditions that take the composition of feed gas into consideration (EP-0882670A1, EP-0926097A1).
For the reactor used for producing the synthesis gas from a gas that contains methane using a ceramic membrane, proposals have been disclosed as concepts of the reactor including the materials (U.S. Pat. No. 5,306,411, WO98/48921, WO99/21640, WO99/21649, EP-0962422A1, EP-0962423A1, U.S. Pat. No. 6,033,632), a method for supplying thermal energy for the reforming of hydrocarbon (WO98/48921), methods for controlling the reaction temperature for partial oxidation of hydrocarbon that is an exothermal reaction (Japanese Unexamined Patent Publication (Kokai) No. 11-70314, U.S. Pat. No. 6,010,614) and a method for sealing the joint between the ceramic membrane and metal (U.S. Pat. No. 5,725,218).
In a membrane reactor wherein synthesis gas is produced, oxygen flows along the flow of gas that contains methane thereby to join there with and, therefore methane concentration tends to be high especially in the upstream thus making carbon precipitation more likely to occur. This takes place in a reaction represented by the following formula.CH4→C+2H2Since the process of producing clean liquid fuel such as Fischer-Tropsch synthesis oil, methanol or dimethyl ether requires the operating pressure of 30 atm or higher, and pressure of the gas including natural gas such as methane must be as high as 50 atm or higher, the operating pressure of the synthesis gas production process is required to be at least 20 atm or higher, that means far higher probability of carbon precipitation than in the case of normal pressure. While the invention of WO99/21649 discloses two-dimensional and three-dimensional arrangements of various methane reforming catalysts, it does not disclose any specific means of suppressing the precipitation of carbon under normal to high pressures. While the invention of EP-0999180A2 discloses a method for suppressing carbon precipitation by recycling the product from the membrane reactor, consisting mainly of hydrogen, to the feed side, it has a drawback of complex process. Meanwhile, it is known that a methane partial oxidation catalyst such as those based on Ni loses the methane reforming capability due to oxidation. As oxygen comes out of the membrane in the membrane reactor, the methane reforming catalyst located near the surface of the membrane tends to be deactivated by oxidation. On the other hand, an increase in partial pressure of oxygen, on the gas side where methane is included that is generated due to decelerated progress of the methane reforming reaction, leads to a significant decrease in the amount of oxygen permeable through the ceramic membrane. In order to put the membrane reactor in commercial applications for producing synthesis gas, it is necessary to provide specific means for solving these problems without compromising the most remarkable feature of the reactor, namely the reactor and related facility can be made compact. However, no basic technologies related to the reaction have been disclosed as to the methane partial oxidation catalyst and the arrangement thereof that would enable practical application, in spite of various technologies related to the reactor that have been proposed so far.
The present invention relates to an oxygen permeation membrane reactor for producing synthesis gas using a gas that contains methane and air as the feedstock, and provides a method for producing synthesis gas with high efficiency, stability and low cost in a range from normal pressure to high pressure around 20 atm, while overcoming such drawbacks as carbon precipitation on the catalyst and deactivation of catalyst due to oxidation of a metal catalyst, and also providing solutions to problems caused by these drawbacks such as a decrease in oxygen supply from the ceramic membrane and the consumption of catalyst.